The present invention relates to an x-ray computed tomography (hereinafter referred to as CT) scanner apparatus, and more particularly, to an arrangement of a member adapted to hold an examined object in position.
2. Description of the Related Art
In an x-ray CT apparatus, e.g., a third-generation x-ray CT apparatus, x-ray fan beams are emitted from an x-ray source while the x-ray source and a multi-channel x-ray detector are being revolved around an examined body, and the beams are detected by means of the detector. The detector delivers a signal proportional to the detected amount of x-rays to a signal processing circuit, whereupon the signal is processed by the processing circuit. Thereafter, projection data are supplied from the signal processing circuit to an image reproducing circuit. Based on these projection data, a slice image of the examined body is reconstructed and displayed on a display unit.
In order to obtain an image of a slice plane of the examined object, data on projection from all directions (360.degree.) for all the area of the slice plane must be prepared. To attain this, the x-ray source and the detector are revolved around a revolution center so that the x-ray fan beams are revolved around the revolution center while the slice plane of the object is being scanned therewith. In this manner, a circle obtained by superposing the revolved fan beams is defined as an effective field of scan.
Thus, if a top plate of a bed unit as well as the examined object, is situated within the effective field, perfect tomography can be effected. However, part of the top plate sometimes may protrude from the effective field. Such a case involves the following problems. Although the fan beams may be transmitted through the protruded portion if the x-ray source is situated in one position, they may not if the x-ray source is situated in another position. If the beams are transmitted through the protruded portion, therefore, the projection data obtained include data on x-rays absorbed by the protruded portion. If the beams are not transmitted through the protruded portion, however, the obtained data do not include the data on the x-rays absorbed by the protruded portion. Accordingly, there are significant differences between the projection data, depending on the transmission of the fan beams. Thus, the resulting reconstructed image sometimes may be subject to shading.
In order to improve the resolution of the image, moreover, the x-ray source and the detector may be shifted around the revolution center to make the effective field of scan relatively small. In this case, part of the top plate is particularly liable to protrude thus entailing the problem of shading.